The invention relates to an irradiation device for emitting ultraviolet radiation, which device comprises a housing in which are arranged a number of parallel extending tubular low-pressure mercury vapor discharge lamps which emit at least ultraviolet radiation having a wave-length of more than 315 nm during operation. The inner wall of each lamp is provided with an ultraviolet reflecting a layer extending over the length of the lamp, but only along part of the tube circumference. Such an irradiation device is known from the Dutch Patent Application No. 7710575 laid open for public inspection.
The aforementioned device is used for photochemical or photobiological applications, for example, in sun beds and solaria, in which the skin of a person situated on or near a transparent wall of the housing is exposed for some time to ultraviolet radiation, especially ultraviolet radiation having a wave-length of more than 315 nm (UV-A radiation).
In order to increase the radiation output of the device, according to the German Offenlegungsschrift No. 2804228, a gutter-shaped reflector is arranged behind each tubular lamp. For a further increase of the radiation output, the number of radiation lamps in the housing could be increased, but it has been found that the relative contribution of the reflectors to the radiation output then decreases strongly. It has further been found that, for example due to the poor ventilation, a comparatively small amount of cool air flows along the lamp walls and the operating temperature of the lamps readily increases to an excessively high value. The mercury pressure then increases to a value exceeding approximately 6.times.10.sup.-3 Torr. At a vapour pressure exceeding 6.times.10.sup.-3 Torr, the efficiency of the conversion of the electric energy supplied to a lamp into resonance radiation of mercury having a wave-length of 254 nm decreases. Additionally, the effectiveness of any ventilator, which is located in the housing of the known device, for example, for cooling purposes and which is often located under the reflectors, is low due to the presence of these reflectors.
The aforementioned Dutch Patent Application discloses an irradiation chamber comprising a system of vertically arranged fluorescent irradiation lamps, each provided with an inner reflecting layer surrounding an irradiation space. Further, reflectors are present on the upper and the lower side of the irradiation space. The lamps are also surrounded by a transparent protecting envelope. In this case, there is a risk that due to the low cooling the mercury vapor pressure in the lamp increases to an excessively high value and the radiation output is reduced. Due to the presence of the envelopes, the number of lamps to be used in the device is comparatively small. Moreover, there is a risk of reduction of the radiation output during operation, due to absorption of radiation by the envelope.